A vehicle is generally said to be “hybrid” when it associates the use of a fuel-burning engine with the use of an electric motor.
In general terms, hybrid vehicles can be operated either by using only the electric motor, or only the engine, or both of them together, depending on the vehicle model.
One particular principle of operation is as follows:                during stationary stages (when the vehicle is not moving), the engine and the motor are both stationary;        on starting, it is the electric motor that causes the car to move, up to higher speeds (25 kilometers per hour (km/h) or 30 km/h);        when higher speeds are reached, the engine takes over;        in the event of hard acceleration, both the engine and the motor are observed to operate simultaneously, thus making it possible to achieve acceleration equivalent to that of an engine having the same power, or even greater acceleration;        during a stage of deceleration and braking, kinetic energy is used for recharging the batteries (it should be observed that this function is not available on all of the hybrid vehicles presently available on the market).        
It results from the above considerations that the engine does not run continuously and that under such circumstances, stages of purging the canister (an activated carbon filter to avoid dumping fuel vapor to the atmosphere) cannot be performed normally since during such stages possibly preheated air is caused to flow through the canister in order to regenerate it (i.e. to desorb the fuel vapor that has been absorbed therein), with this air then being admitted into the engine where it is burnt.
Under such circumstances, in order to avoid pointlessly loading the canister, communication between the tank and the canister is generally blocked by default; as a result the fuel tanks of such vehicles are generally put under pressure (typically pressure of about 300 mbar to about 400 mbar), with this generally being achieved by means of a functional element situated after the ventilation valves, often referred to as the fuel tank isolation valve (FTIV), that prevents the tank being ventilated (degassed) other than during filling situations. Such an element generally comprises two safety valves (rated at the lowest and highest pressures at which the tank can be used) with control that is generally electrical in order to enable the tank to be put to atmospheric pressure before it is filled.
The positioning of the FTIV in the system leads to problems in practice:                if it is positioned upstream from the canister, it avoids vapors being sucked in from the tank while the canister is being purged, but in contrast it splits the fuel system into two zones, which makes it more difficult to comply with the tests required by the on-board diagnosis (OBD) regulations; and        when positioned downstream from the canister, the situation is reversed: it facilitates OBD testing but it does not prevent vapor being sucked in from the tank, with this applying even though from the point of view of canister loading, there is no difference between the two positions (since even in the downstream position, given that there is no flow, there is no loading on the canister even if it is connected to the tank).        
One solution to that problem consists in using two separate valves, one valve that is normally closed but that is opened to provide ventilation during filling, and another valve that serves to disconnect the tank from the canister during purging. By way of example, such a solution is described in U.S. Pat. No. 6,167,920 and U.S. Pat. No. 7,448,367, however it is expensive and generally requires recourse to electronics. Using mechanical control would increase the reliability and the robustness of the solution, would simplify it, and would reduce its costs, given the absence of electronics.